1. Field of the Invention
The present invention relates to a process for preparing a fluorine-containing copolymer. More particularly, the present invention relates to a process for preparing a fluorine-containing copolymer in which the concentration of metal elements, which will be an impurity source, is lowered.
The present invention further relates to a fluorine-containing copolymer having a lowered concentration of metal elements, which will be an impurity source, and suitable, for example, for transportation rolls, sealing materials, hoses, tubes and the like in the field of liquid crystal and semiconductor production apparatuses, or polymer matrixes for forming polymer electrolytes of lithium secondary batteries in the electric field, and also relates to cross-linking (vulcanization) molded articles thereof.
2. Description of the Prior Art
Fluorine-containing copolymers generally tend to have enhanced chemical resistance, heat resistance and higher purity as compared with polyolefins or the like and have been conventionally used, for example, for molded articles such as transportation rolls, sealing materials, hoses, tubes and the like in the field of liquid crystal and semiconductor production apparatuses, or polymer matrixes for forming polymer electrolytes of lithium secondary batteries in the electric field.
In the liquid crystal and semiconductor production processes, impurities contained in the above molded articles formed from the fluorine-containing copolymers such as metal elements and the like are called particles and cause wafer surface contamination and thereby induce product defects. Therefore, the removal of metal elements remaining is an important subject with the view of improving the yield of products.
Accordingly, with regard to materials for use in liquid crystal and semiconductor production apparatuses, it is also desired to use materials incapable of generating impurity substances, which induce product defects.
In polymer electrolytes used for lithium ion secondary batteries, the inclusion of metal elements is undesirable because it is considered that the transfer of lithium ion will be interfered or other facts will be caused, to thereby induce a lowering of an ion conductivity of the polymer electrolyte or to induce a lowering of a discharge capacity in the cycle of charging and discharging.
Conventionally, fluorine-containing copolymers are prepared in the following manner. A fluorine containing copolymer dispersed aqueous solution synthesized by an emulsion polymerization method using a fluorine-containing anionic surfactant was dropped in an aqueous solution in which an inorganic salting out agent such as sodium chloride alone, or plural kinds of ones such as sodium chloride, potassium alum, magnesium chloride and the like with stirring and thereby the fluorine-containing copolymer contained therein is aggregated and isolated.
The salting out method, however, has a defect such that as the inorganic salting out agents used in salting out are contained and adsorbed in the fluorine containing copolymer together with aggregation of the fluorine containing copolymer, it is difficult to completely remove the inorganic salting out agents contained in the fluorine containing copolymer even if washing thereof with pure water is carried out repeatedly, and further fluorine containing copolymers having a high content of metal elements are undesirably prepared due to the inorganic salting out agent used.
As a method for isolating the fluorine containing copolymer without using a salting out agent, there is a freeze-coagulation method of carrying out coagulation by freezing a fluorine-containing copolymer dispersed aqueous solution. The fluorine containing copolymer prepared by this method has adsorbed and contained an anionic surfactant and a pH modifier used in the preparation of the copolymer so that the copolymer is disadvantageous for decreasing impurities and is not preferable in viewpoint of productivity.
As a method of isolating the fluorine containing copolymer without using the salting out method, there are a solution polymerization method and a solution suspension polymerization method.
In the case of preparing the fluorine containing copolymer in an elastomer region, it is advantageous to employ the emulsion polymerization method in view of the reactivity of the fluorine containing monomers and batch efficiency thereof.
The present invention solves the problems associated with the prior art by providing a process for preparing a fluorine containing copolymer which produces a high purity fluorine containing copolymer having a lowered concentration of metal elements from a fluorine containing copolymer dispersed aqueous solution prepared by an emulsion polymerization method, preferably an emulsion polymerization method without using metal element containing starting materials.
The present invention further provides a fluorine containing copolymer suitable, for example, for transportation rolls, sealing materials, hoses, tubes and the like in the field of liquid crystal and semiconductor production apparatuses, or polymer matrixes for forming polymer electrolytes of lithium secondary batteries in the electric field, wherein the uses are obtainable by melt molding or vulcanization (cross-linking) molding the fluorine containing copolymer prepared by the above method and have a lowered concentration of metal elements.